Attitude can be defined as the inclination of a user platform relative to a reference frame in three-dimensional space. The technologies traditionally used for attitude determination can be categorized into two groups. The first group provides absolute attitude measurements with respect to a reference. Examples of this kind of technology are magnetic field sensors, optical trackers, and star sensors. The second group can only provide measurements of changes in attitude. Examples are gyroscopes (mechanical, MEMS, ring laser and fiber optic) and angular accelerometers. Collectively, devices in this second group are known as inertial devices. Applications using inertial devices require the initial attitude to be known. This is normally achieved using technologies from the first group. Only when an initial attitude is available to the application can subsequent absolute attitude be determined from measurements provided by the second group.
Unfortunately, many problems exist in using devices from either group. The calculated attitude derived from inertial devices suffers from continuous drift over time due to accumulated measurement errors in angular rates, and therefore an inertial system must be frequently realigned to a reference if it is to be useful. An optical tracker's operation is limited by the field of view of its sensors, which must be unobstructed for normal operation. Star sensors, which involve star pattern recognition, have the added limitation of only being operational on cloudless nights for earth based systems. Determining orientation using magnetic field sensors relies on the integrity of the reference magnetic field. This field is often distorted, for example by large ferrous objects in proximity to the magnetic field sensor, rendering performance unsatisfactory in environments like large industrial areas, factories and warehouses.
An alternative to the traditional methods of attitude determination described above is an extension of the carrier-phase positioning techniques used by Global Positioning System (GPS) receivers. Carrier-phase positioning determines position by determining the Doppler induced upon received positioning signals when a user receiver is in motion. The user receiver accumulates these Doppler measurements over time, thereby producing highly accurate change-of-range measurements known in the art as accumulated Doppler, or Integrated Carrier Phase (ICP) measurements. Integrated Carrier Phase (ICP) measurements are processed by the user receiver to derive accurate change-of-range measurements from a plurality of satellites at known locations, which in turn can be used to determine accurate position relative to a known initial location. This initial location is generally derived from code-based pseudorange measurements which are modulated upon the carrier signals transmitted from each satellite. Due to the lack of precision inherent in the code-based pseudoranges, a cycle ambiguity remains when determining this initial starting location. This cycle ambiguity must be resolved, using techniques well-known in the art, before high-accuracy positioning can take place. Although carrier-phase positioning systems work well for precise position determination, they possess no inherent means for determining attitude.
Prior art systems have attempted to overcome this limitation. In particular, U.S. Pat. No. 5,548,293 in the name of Cohen discloses a carrier-phase based positioning system wherein attitude is determined by spatially distributing a plurality of GPS receivers on a mobile platform and concurrently collecting carrier-phase range measurements from all GPS satellites in view. Carrier-phase range differences are subsequently calculated between GPS receivers to determine the attitude of the user platform. Cohen's method is not only complicated and costly due to the number of carrier-phase GPS receivers required, but it also requires integer cycle ambiguities to be resolved before attitude can be determined. Furthermore, the use of GPS signals requires the plurality of GPS receivers to be in clear view of the satellites at all times, thus eliminating the ability of the system to operate in satellite-occluded environments, such as indoors. Moreover, the reflection of positioning signals, known as multipath, degrades range measurement accuracy from each satellite, which therefore degrades attitude determination when using Cohen's method.
There is clearly a need for a robust absolute attitude determination system that does not require (a) reference magnetic field integrity, (b) an unobstructed view of the stars, (c) line-of-sight for optical measurements, (d) continual re-initialization, (e) an unobstructed view of a Global Navigation Satellite System (GNSS), (f) multiple position receivers distributed over a user platform, or (g) the requirement for carrier cycle integer ambiguity resolution. A system that operates without these constraints is highly desirable. The present invention achieves this desirable goal by transmitting signals that are modulated with a three-dimensional spatial signature, modulating a receiving means with a complementary three-dimensional spatial signature, and interpreting the received signals to determine attitude. This system and method is hereinafter termed Spatial Shift Key (SSK) modulation, and is described in detail below.